Not applicable.
The invention concerns a method and a device for measurement and regulation of the retention profile of the web in a paper/board machine.
The invention is related to the dilution liquid system in a paper machine and more specifically to measurement and control of the retention profile of the web in the cross direction of the machine. Further, by means of the device in accordance with the invention, it is possible to locate various situations of malfunction in the wire part of a paper machine or equivalent.
A dilution headbox is known from the applicant""s Patent Applications FI-901593, FI-933027 and FI-942780 of earlier dates. In a dilution headbox, the basis weight of the web is regulated so that a dilution flow is passed through a valve to different areas of width of the headbox and so that the quantity of said flow is regulated. The dilution flow is mixed with the stock flow passed from the inlet header of the headbox. As the dilution liquid, it is possible to use pure water or, for example, filtrate water returned from the web. In the applicant""s Patent FI 92,229, a construction of a three-way dilution valve is described, which is used for regulation of the dilution flow.
As is known from the prior art, a stock suspension jet is discharged out of the slice opening in paper or board machines onto a forming wire or into a gap between wires. The proportion of the solid matter that remains on the wire, i.e. the retention, consists of a fibre retention, whose proportion is about 60-80%, and of a filler retention, whose proportion is about 20-40%. The filtrate passing through the wire, i.e. the so-called white water, contains an abundance of fibrous material and fillers, and it is returned back to the process of manufacture. By means of measurement of the properties of the white water, information is obtained, among other things, on the success of the dilution regulation. When the measurement is carried out at a number of points in the cross direction, it is possible to conclude the distribution of fibres and fillers in the cross direction of the web. Attempts are made to regulate the retention of fibres and the retention of fillers so that they are distributed as uniformly as possible in the cross direction of the machine.
Factors that affect the retention include, among other things, headbox consistency, construction of the wire part, and properties of the stock, such as distribution of fibre length, fillers, and added chemicals. Measurement of basis weight does not give a correct picture of the retention profile, for with a uniform basis weight profile, the fibre retention profile and the filler retention profile can, nevertheless, be uneven. This is why determination of the retention profile in the wire part would give a correct idea of the retention profiles.
For regulation of the retention, a retention agent is employed, whose function is to bind fillers and fines to the fibres in order that said agents should not depart from the web through the holes provided in the wire. The particle size of fillers and fines is considerably smaller than the size of the holes provided in the wire.
The object of the invention is to provide a method of measurement for measurement of the retention profile.
An object of the invention is to provide a method of regulation for regulation of the retention profile in the cross direction.
A further object of the invention is to provide a device for measurement of the retention profile.
An object of the invention is to provide a device for regulation of the retention profile.
The method in accordance with the invention is characterized in that, in the method, from the white water drained from the web through the wire in the wire part, samples are taken by means of a sample collecting device from a number of points in the cross direction of the web, preferably at uniform distances, the samples that were taken are analyzed by means of an analyzer, a retention profile is formed out of the measurement results, and the retention profile thus formed is used for controlling the paper machine.
The device in accordance with the invention is characterized in that the device comprises means for measurement of the retention profile across the web width from different points of width of the web out of the white water drained from the wire part, and that the headbox comprises lines passing to different points of width of the headbox so as to pass a flow of liquid that contains a retention agent in the headbox of the paper or board machine into connection with the stock flow so as to regulate the retention profile across the web width, and that the lines comprise, in their connection, a valve construction for regulation of the amount of retention agent into the flow moving in the line, the regulation of the valves taking place on the basis of the measurement data obtained from measurement of the retention profile.
In the present invention, an arrangement is suggested for measurement and regulation of the retention profile in the cross direction of the machine. A white water sample is collected into a sampling vessel and transferred to an analyzer. The analyzer can also be integrated in a measurement head. After taking of the sample and after its transfer to the analyzer, the measurement head is transferred to the following measurement point. With this procedure, the retention profile can be determined across the width of the whole machine direction. Samples can be taken, for example, at intervals of 10 cm in the machine direction.
The determined retention profile is utilized in the regulation of the dilution liquid system, in which connection each dilution liquid valve can be provided with the necessary regulation of the concentration of retention agent. A measurement arrangement accomplished in this way operates as on-line measurement, and it can be carried out constantly during the whole run. The measurement also provides information on problem situations, e.g. on blocking or contamination of the wire fabric, on incomplete mixing of retention agent, or on other situations of failure.
Regulation of the retention profile is carried out so that the basis weight profile is not changed. In such a case, a change in the basis weight of the paper arising from a change in the quantity of retention agent is compensated for by means of a change in the dilution quantity. For example, if a local addition of retention agent increases the basis weight, the dilution quantity at said point is increased.
In the following, the invention will be described in more detail with reference to the exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing, the invention being not supposed to be confined to the details of said embodiments alone.